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Full text of "Text Book Of Mechanical Engineering"

Combustion.

697

Taking the atomic weights of C, H, and O, as 12,  i, and 16
respectively, we have :

Marsh gas  +   Oxygen   = Carbon dioxide +     Water

(12 + 4)   +2(16x2)==   {i2 + (i6x2)}   +2(2 + 16)

that is,      i61bs.     +    64lbs.    =         44lbs.         +    36 Ibs.

or,             ilb.      +      4 Ibs. gives    275 Ibs.         + 2-25 Ibs.

Again, i Ib. of carbon burnt to C02 gives 14,500 thermal
units, and i Ib. of hydrogen burnt to H2 O gives 62,032 units,
In i Ib. of marsh gas there is  Ib. of carbon and \ Ib. of
hydrogen.

Units.

. -fib. Carbon + O      gives  14,500 x  = 10,875
Jib. Hydrogen + O  gives 62,032 x \ = 15,508

Total

26,383

Experimentally we obtain a total of  23,582 units, or   2801
units has been required for decomposing the C and H.

Good dry bituminous coal contains on the average, by weight,

Carbon, 83-5 %      Hydrogen, 4-6 %      Oxygen, 3*15 %

the remaining 8*75 % being Nitrogen and Sulphur, inactive
elements. Taking 100 Ibs. of fuel the 3*15 Ibs. of oxygen is
already united to -J- x 3*15 = '4lb. of hydrogen as water, and the
hydrogen does not assist combustion; so we have left:

83*5 Ibs. of Carbon

4*2 Ibs. of Hydrogen

Now 12 Ibs. of C unite with 32 Ibs. of O, or as i : 2'66 ; and
2 Ibs. of H require 16 Ibs. of 0, or as i : 8.

Ibs. of O.

.*. 83*5 Ibs. C require 83-5 x 2*66 = 222
and 4*2 Ibs. H require   4*2 x 8      =   33*6

Total weight Oxygen for loolbs. coal = 255'6 Ibs.

or 2-5 Ibs. of Oxygen is needed to burn i Ib. of coal.    But air is
composed of 77 parts Nitrogen to 23 of Oxygen, by weight.

/.   23 : 100 :: 2*5 = 10 Ibs. of air per Ib. of fuel.

i